1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for adaptively determining a transmit antenna diversity scheme according to a channel condition.
2. Description of the Related Art
Generally, high speed downlink packet access (hereinafter referred to as “HSDPA”) is the general term for a high speed downlink shared channel (hereinafter referred to as “HS-DSCH”) which is a downlink data channel for supporting high speed downlink packet data transmission, its associated control channels, and an apparatus, system and method therefor in a wideband code division multiple access (hereinafter referred to as “W-CDMA”) communication system. Although the present invention will be described with reference to HSDPA in 3GPP (3rd Generation Partnership Project), which is a standard of a 3rd generation asynchronous mobile communication system. The invention can also be applied to other communication systems which realize transmit diversity by using two or more transmit antennas.
In a communication system using HSDPA, adaptive modulation and coding (hereinafter referred to as “AMC”), hybrid automatic retransmission request (hereinafter referred to as “HARQ”), and fast cell select (hereinafter referred to as “FCS”) have recently been introduced to support high speed packet data transmission.
AMC refers to a data transmission scheme for adaptively determining a modulation scheme and a coding scheme of a data channel according to a channel condition between a particular Node B and a user element (UE) thereby improving the overall utilization efficiency of the cell. AMC has a plurality of modulation schemes and coding schemes, and modulates and codes a data channel signal by combining the modulation schemes and coding schemes. Usually, each combination of the modulation schemes and the coding schemes is referred to as “modulation and coding scheme (MCS)”, and a plurality of MCSs of a level #1 to a level #n can be defined according to the number of the MCSs. That is, AMC is a technique for improving overall system efficiency of a Node B by adaptively determining an MCS level according to a channel condition between a UE and a Node B wirelessly connected to the UE.
Second, N-channel stop and wait hybrid automatic retransmission request (hereinafter referred to as “N-channel SAW HARQ”), typical HARQ, will be described. In common automatic retransmission request (hereinafter referred to as “ARQ”), an acknowledgement (hereinafter referred to as “ACK”) signal and retransmission packet data are exchanged between a UE and a radio network controller (RNC). For HARQ, the following two proposals have been recently provided in order to increase transmission efficiency of ARQ. As a first proposal, HARQ exchanges a retransmission request and a retransmission response between a UE and a Node B. As a second proposal, HARQ temporarily stores defective data and then combines the defective data with its retransmitted data before transmission. Further, in HSDPA, an ACK signal and retransmission packet data are exchanged between the UE and the Node B over a medium access control (MAC) HS-DSCH. Moreover, HSDPA has introduced N-channel SAW HARQ in which N logical channels are formed to transmit several data packets before an ACK signal is received. However, in SAW (Stop and Wait) ARQ, next packet data is not transmitted until an ACK signal for previous packet data is received. Therefore, in some cases, a UE or a Node B must wait for an ACK signal even though the UE or Node B can currently transmit packet data. However, in N-channel SAW HARQ, a UE or a Node B can continuously transmit packet data even before an ACK signal for previous packet data is received, thereby increasing channel efficiency. That is, N logical channels are set up between a UE and a Node B. If the N logical channels can be identified by time or a channel number, a UE receiving packet data can determine a logical channel over which the packet data is received. In addition, the UE can reconfigure the packet data in the right order or soft-combine the corresponding packet data.
Finally, FCS will be described. In FCS, if a UE supporting HSDPA is located in a cell overlapping region, or a soft handover region, a cell having the best channel condition is selected from a plurality of cells. Specifically, if a UE supporting HSDPA enters a cell overlapping region between a first Node B and a second Node B, the UE sets up radio links to a plurality of cells, or Node Bs. A set of the cells to which the UE sets up radio links is referred to as “active set.” The UE receives HSDPA packet data only from a cell having the best channel condition among the cells included in the active set, thereby reducing overall interference. Herein, the cell having the best channel condition will be referred to as “best cell.” In order to determine the best cell, the UE must periodically monitor channel conditions of the cells included in the active set to determine whether there is any cell having a better channel condition than the current best cell. If there is any cell having a better channel condition, the UE transmits a best cell indicator to all cells belonging to the active set in order to replace the current best cell with the new best cell. The best cell indicator includes an identifier of the new best cell. Each cell in the active set receives the best cell indicator and analyzes a cell identifier included in the received best cell indicator. That is, each cell in the active set determines whether a cell identifier included in the best cell indicator is identical to its own cell identifier. If the cell identifiers are identical to each other, the corresponding cell selected as a new best cell transmits packet data to the UE over HS-DSCH.
As described above, the communication system using HSDPA proposes various new schemes in order to increase a data rate. In addition to the above-mentioned new schemes of AMC, HARQ and FCS, there is a transmit antenna diversity scheme for coping with a fading phenomenon on a radio channel as another scheme for increase a data rate. The transmit antenna diversity scheme refers to a technique for minimizing a transmission data loss due to a fading phenomenon. The signal is transmitted using two or more transmit antennas, thereby increasing a data rate. The transmit antenna diversity scheme will now be described herein below.
Generally, in a radio channel environment in a mobile communication system, unlike a wired channel environment, a signal is received distorted due to various causes such as multipath interference, shadowing, wave attenuation, time-varying noise and time-varying interference, and the like. Fading caused by the multipath interference is related to a reflecting substance or mobility of a user, i.e., a UE, and an actual transmission signal is mixed with an interference signal during reception. Therefore, the reception signal is a transmission signal which suffers from severe distortion. Fading acts as a main cause of deterioration of the performance of a mobile communication system. As a result, since fading can distort amplitude and a phase of a received signal, it is a chief cause of interference for high-speed data communication in a radio channel environment. Thus, research has been performed on he fading phenomenon.
As an effective scheme for solving the fading problem, the transmit antenna diversity scheme attracts public attention. The transmit antenna diversity scheme receives a plurality of transmission signals which suffer from independent fading phenomenon in a radio channel environment, and copes with distortion due to the fading. The transmit antenna diversity scheme includes a time diversity scheme, a frequency diversity scheme, a multipath diversity scheme, and a space diversity scheme. The time diversity scheme effectively copes with a burst error occurring in a radio channel environment by using an interleaving technique. In the frequency diversity scheme, signals transmitted at different frequencies have different multipaths, obtaining diversity gain. The multipath diversity scheme achieves diversity by separating multipath signals since the multipath signals have different fading information. In the space diversity scheme, a Node B or a UE transmits and receives signals by using a plurality of antennas so that the transmission and reception signals experience independent fading, thereby obtaining diversity gain.
The space diversity scheme uses a plurality of transmit and receive antennas. In the space diversity scheme, a Node B generally includes two or more transmit antennas to improve performance of a radio link. Also, a UE can include two or more receive antennas to improve radio link performance. However, the UE has many limitations such as power consumption, miniaturization, lightweightness and complexity, so the space diversity scheme is generally applied to a Node B. For these reasons, a Node B transmits a signal with a plurality of transmit antennas, and a UE receives a signal with one receive antenna. However, there have been proposed various plans to cope with fading of a radio channel by generating diversity gain similar to that in the case where the UE has a plurality receive antennas. In particular, methods of realizing space diversity by using two or more transmit antennas, for a next generation mobile communication system proposed in 3GPP, attracts public attention. A space diversity scheme proposed for the next generation mobile communication system includes a space time transmit diversity (hereinafter referred to as “STTD”) scheme which is an open-loop type transmit antenna diversity scheme using space-time coding without state information of a radio channel, and a transmit antenna array (hereinafter referred to as “TxAA”) scheme which is a closed-loop transmit antenna diversity scheme using state information of a radio channel, fed back from a UE.
With reference to FIG. 1, a description will now be made regarding a structure of a transmission apparatus that transmits data by using an STTD scheme which is the closed-loop scheme.
FIG. 1 is a block diagram illustrating an example of a data transmission apparatus using an STTD scheme. Referring to FIG. 1, the data transmission apparatus, i.e., a Node B's transmission apparatus, includes an STTD encoder 20 for STTD-encoding input symbols, i.e., input data being subjected to a series of data processing processes such as channel coding and interleaving, a channelization code/scrambling code generator 26 for generating a channelization code and a scrambling code corresponding to each of STTD-encoded symbols output from the STTD encoder 20, multipliers 22 and 24 for multiplying the channelization code and the scrambling code generated from the channelization code/scrambling code generator 26 by the STTD-encoded symbols, multipliers 28 and 30 for multiplying signals output from the multipliers 22 and 24 by corresponding transmission power, and antennas 32 and 34 for transmitting signals output from the multipliers 28 and 34, respectively.
The structure of the data transmission apparatus will now be described in detail herein below.
First, symbols x1 and x2 are applied to the STTD encoder 20 after being subjected to transmission data processing processes such as channel coding and interleaving. The STTD encoder 20 then STTD-encodes the input symbols x1 and x2. A method for STTD-encoding the input symbols x1 and x2 in the STTD encoder 20 will be described below. The input symbols x1 and x2 are converted into encoded symbols (x1,x2) and (−x2*,x1*) through an STTD encoding process given by
                                                                        Time                ⁢                                                                  ⁢                Symbol                ⁢                                                                  ⁢                1                                                                                        Time                ⁢                                                                  ⁢                Symbol                ⁢                                                                  ⁢                2                                                    ⁢                              (                                                                                                      x                      1                                        -                                          x                      2                      *                                                                                                                                                              x                                              2                        ⁢                                                                                                                                        ⁢                                          x                      1                      *                                                                                            )                                Ant            ⁢                                                  ⁢            1            ⁢                                                  ⁢            Ant            ⁢                                                  ⁢            2                                              (        1        )            
The STTD encoder 20 provides the encoded symbols (x1,x2) and (−x2*,x1*) to the multipliers 22 and the 24, respectively. The multiplier 22 multiplies the encoded symbol (x1,x2) output from the STTD encoder 20 by a channelization code and a scrambling code output from the channelization code/scrambling code generator 26, and provides its output to the multiplier 28. The multiplier 24 multiplies the encoded symbol (−x2*,x1*) output from the STTD encoder 20 by a channelization code and a scrambling code output from the channelization code/scrambling code generator 26, and provides its output to the multiplier 30. The multiplier 28 multiplies a signal output from the multiplier 22 by transmission power √{square root over (P/2)} assigned to the (x1,x2) and transmits the result signal through the antenna 32. The multiplier 30 multiplies a signal output from the multiplier 24 by transmission power √{square root over (P/2)} assigned to the (−x2*,x1*) and transmits the result signal through the antenna 34.
The signals transmitted via the antennas 32 and 34 are received at a data reception apparatus, i.e., a UE's reception apparatus, and the signals received by the data reception apparatus are expressed asr1=h1x1−h2x2*+n1r2=h1x2+h2x1*+n2  (2)
In Equation (2), r1 and r2 represent reception signals at a corresponding reception time, h1 and h2 represent channel responses of the antennas 32 and 34, respectively, and n1 and n2 represent additive white Gaussian noises (hereinafter referred to as “AWGN”). The UE's reception apparatus then restores the reception signals received in the form of Equation (2) to the original transmission signals transmitted from the Node B's transmission apparatus, through a demodulation process expressed as{circumflex over (x)}1−r1h1*+r2*h2−(|h1|2+|h2|2)x1{circumflex over (x)}2−−r1*h2+r2h1*−(|h1|2+|h2|2)x2  (3)
As a result, the UE's reception apparatus achieves diversity gain by combining independent fading components from the antennas according to the demodulation process.
The STTD scheme, an open-loop scheme, has been described so far with reference to FIG. 1. Next, feedback information used in the closed-loop transmit diversity scheme will be described with reference to FIG. 2.
FIG. 2 is a diagram illustrating an example of general feedback information used in a closed-loop transmit diversity scheme. Referring to FIG. 2, the feedback information is transmitted from a UE to a UTRAN (UMTS Telecommunication Radio Access Network). For example, the UE transmits the feedback information through a feedback information (FBI) field (not shown) of a dedicated physical control channel (hereinafter referred to as “DPCCH”). The feedback information will be described herein below. The feedback information is comprised of an Nph-bit feedback signaling message (hereinafter referred to as “FSM”) field i.e., an FSMph field, representing phase information, and an Npo-bit FSM field, i.e., an FSMpo field, representing power information.
The feedback information used in the closed-loop transmit diversity scheme has been described so far with reference to FIG. 2. Next, a structure of a transmission apparatus that transmits data by using a TxAA scheme which is a closed-loop scheme will be described with reference to FIG. 3.
Before a description of FIG. 3, the TxAA scheme will be described below. An operation mode of the TxAA scheme is roughly divided into a first TxAA mode (hereinafter referred to as “TxAA Mode1”) and a second TxAA mode (hereinafter referred to as “TxAA Mode2”). First, the TxAA Mode1 will be described. In the TxAA Mode1, a UE calculates weights w1 and w2 to be used in a UTRAN by using a pilot signal transmitted from a Node B so that reception power of a received signal can be maximized. That is, the UE calculates a relative phase difference between a first antenna ANT1 and a second antenna ANT2 for each slot, quantizes the calculated phase difference, and transmits the quantized phase difference to a Node B. The phase difference is expressed in two values of π and 0, and the UE sets the phase difference to 1 and 0 through the FSMph field, and transmits the set phase difference to a UTRAN, i.e., Node B.
The Node B calculates a relative phase difference φ1 of the second antenna ANT2 by using Table 1 below, for an FSMph value of each slot. Thereafter, the Node B calculates a weight vector of the second antenna ANT2 by using the φ1 in accordance with
                                          w            1                    =                      1                          2                                      ,                              w            2                    =                                                                      ∑                                      i                    =                                          n                      -                      1                                                        n                                ⁢                                                                  ⁢                                  cos                  ⁡                                      (                                          ϕ                      i                                        )                                                              2                        +                          j              ⁢                                                                    ∑                                          i                      =                                              n                        -                        1                                                              n                                    ⁢                                                                          ⁢                                      sin                    ⁡                                          (                                              ϕ                        i                                            )                                                                      2                                                                        (        4        )            
TABLE 1Slot #i01234567891011121314SFM00  π/20  π/20  π/20  π/20  π/20  π/20  π/201π−π/2π−π/2π−π/2π−π/2π−π/2π−π/2π−π/2π
Next, the TxAA Mode2 will be described herein below. Unlike the TxAA Mode1, the TxAA Mode2 adjusts both phase and amplitude, i.e., power information. That is, although the TxAA Mode1 adjusts only phase, the TxAA Mode2 adjusts not only the phase but also amplitude. Currently, the total number of weights available in the UE is 16, and each of the 16 weights has a value distinguished into phase and amplitude. The feedback information distinguished into phase and amplitude, i.e., FSMph and FSMpo, are shown in Table 2 and Table 3 below.
TABLE 2FSMpoPower_ant1Power_ant200.20.810.80.2
TABLE 3FSMphPhase difference between antennas (radians)000π001−3π/4011 −π/2010 −π/41100111   π/4101   π/2100  3π/4
Table 2 illustrates values of FSMpo. For example, when a value of FSMpo is set to 0, if amplitude Power_ant1 of a first antenna is 0.2, then amplitude Power_ant2 of a second antenna is set to 0.8. Table 3 illustrates values of FSMph. For example, when a value of FSMph is set to 000, a phase difference between antennas (radian) is set to π.
Thus, weight vectors of the first and second antennas are calculated by
                                          w            …                    =                      [                                                                                                      power_ant                      ⁢                                                                                          ⁢                      1                                                                                                                                                                                      power_ant                        ⁢                                                                                                  ⁢                        2                                                              ⁢                                          exp                      ⁡                                              (                                                  j                          ⁢                                                                                                          ⁢                          phase_diff                                                )                                                                                                                  ]                          ⁢                                  ⁢                                  ⁢        8                            (        5        )            
The data transmission apparatus of FIG. 3 will now be described herein below.
FIG. 3 is a block diagram illustrating an example of a data transmission apparatus using the TxAA scheme. Referring to FIG. 3, the data transmission apparatus, i.e., a Node B's transmission apparatus, includes a channelization code/scrambling code generator 44 for generating a channelization code and a scrambling code corresponding to each of input symbols, i.e., input data being subjected to a series of data processing processes such as channel coding and interleaving, multipliers 40 and 42 for multiplying the channelization code and the scrambling code generated from the channelization code/scrambling code generator 44 by the input symbols, multipliers 46 and 48 for multiplying signals output from the multipliers 40 and 42 by corresponding weights, multipliers 50 and 52 for multiplying signals output from the multipliers 46 and 48 by a corresponding power, and antennas 54 and 56 for transmitting signals output from the multipliers 50 and 52, respectively.
The structure of the data transmission apparatus will now be described in detail below.
First, symbols (x1,x2) are applied to the multipliers 40 and 42 after being subjected to transmission data processing processes such as channel coding and interleaving. The multiplier 40 multiplies the input symbols (x1,x2) by a channelization code and a scrambling code output from the channelization code/scrambling code generator 44, and then provides its output to the multiplier 46. The multiplier 42 multiplies the input symbols (x1,x2) by a channelization code and a scrambling code output from the channelization code/scrambling code generator 44, and then provides its output to the multiplier 48. The multiplier 46 multiplies a signal output from the multiplier 40 by a corresponding weight w1, and then provides its output to the multiplier 50. The multiplier 50 multiplies a signal output from the multiplier 46 by corresponding transmission power √{square root over (P/2)}, and then transmits the resulting signal via the antenna 54. The multiplier 48 multiplies a signal output from the multiplier 42 by a corresponding weight w2, and then provides its output to the multiplier 52. The multiplier 52 multiplies a signal output from the multiplier 48 by corresponding transmission power √{square root over (P/2)}, and then transmits the result signal via the antenna 56.
The transmit antenna diversity schemes described above show different performances according to a speed (or rate) of a fading channel, i.e., a variation speed of a fading channel.
For example, if a moving speed of a UE is lower than 20 Km/h, the TxAA Mode2 scheme achieves the best diversity gain, and if a moving speed of the UE ranges from 20 Km/h to 70 Km/h, the TxAA Mode1 scheme shows the best diversity gain. If a moving speed of the UE is higher than 70 Km/h, the STTD scheme shows the best diversity gain. As mentioned above, since a transmit antenna diversity scheme capable of maximizing diversity gain is different according to a moving environment of the UE or a condition of a radio channel, there is a demand for a method of selecting the most appropriate transmit antenna diversity scheme according to circumstances.